Mist Chemical Vapor Deposition Research Articles

Heteroepitaxial growth of Ga2O3 thin films on nickel-nanodot-induced buffer layers for solar-blind ultraviolet photodetector applications

Heteroepitaxial growth of gallium oxide (Ga2O3) thin films has been conducted on nickel (Ni)-nanodot-induced buffer layers using a mist chemical vapor deposition. The separation degree and droplet size of the Ni nanodots on sapphire substrates were controlled through a thermal annealing process, thereby being used as templates for the Ga2O3 thin-film growth. The growth of the inverted cone-shaped ε-Ga2O3 polymorph was observed on the nanodots while the α-Ga2O3 polymorph was grown on the sapphire substrate without nanodots, ultimately a thin film with a mixture of ε and α polymorphs can be achieved. By comparing photoresponses of metal–semiconductor–metal photodetectors fabricated on the thin film with the mixed polymorphs and on a single-crystalline α-Ga2O3 film, a promising template for using solar-blind photo-detecting applications has been verified.

High-quality Cr2O3 - Ga2O3 solid solutions films grown by mist-CVD epitaxy

Micron-thick layers of (Cr1-xGax)2O3 solid solutions were grown by modified mist chemical vapor deposition (mist-CVD) with three different Ga concentrations. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) methods were used to analyze the quality of the films. They showed good crystallinity, homogeneity and coalescence of the samples. Solid solution contents estimation was performed via applying Vegard’s law to XRD data and by its results the highest reached Cr:Ga ratio is approximately 1:1. Transmission spectra of solid solutions demonstrated blue-shift of the absorption edge with increase of the Ga contents. Optical bandgap increased from 3.06 eV for undoped Cr2O3 sample to 3.73 eV for the layer with the highest Ga concentration.

Improvement of Photoconductivity in a‐Oriented α‐Ga2O3 Thin Films Grown on Sapphire Substrates by Mist Chemical Vapor Deposition

α‐Ga2O3 is a suitable material for UV‐C optical devices owing to its optical absorption edge wavelength. In this study, α‐Ga2O3 thin films are grown on c‐, a‐, m‐, n‐, and r‐oriented sapphire substrates by mist chemical vapor deposition. Furthermore, their structural fluctuations, (normal direction of the surface) and (rotational direction on the surface), are examined. As a result, the a‐oriented α‐Ga2O3 thin films exhibit the smallest and . Based on the results of the previous examination, metal–semiconductor–metal (MSM) photodetectors are fabricated using c‐ and a‐oriented α‐Ga2O3 thin films and their photoconducting properties are characterized. Under D2 lamp light illumination, the MSM photodetector using a‐oriented films produces photocurrent four to six times greater than those using c‐oriented films. The visible‐light rejection ratios are at 10 V and 105.2 at 24 V. The photoresponsivity is estimated to be 2.2 A W−1 under the illumination of a D2 UV lamp and 24 V bias voltage. In these results, it is suggested that the a‐oriented α‐Ga2O3 thin film exhibits a higher in‐plane carrier mobility than the c‐oriented film. Thus, a‐oriented α‐Ga2O3 films are more suitable than c‐oriented α‐Ga2O3 films for fabricating MSM photodetectors.

Nitrogen Doping in VO2 Thin Films on Synthetic Mica Substrates Through Mist Chemical Vapor Deposition: Lowering the Metal–Insulator Transition Temperature Toward Smart Windows

AbstractIn this study, nitrogen (N) is doped into VO2 thin films through mist chemical vapor deposition (CVD), and the effect of the doping on metal–insulator transition (MIT) temperatures is investigated. The N‐doped VO2 thin films are grown on an SnO2 buffer layer. The N‐doped VO2 lattice spacing tends to expand as the growth temperature decreased, which indicates that the incorporation of N into the lattice is derived from the Ethylenediamine. Secondary ion mass spectrometry (SIMS) is conducted to investigate the relationship between the decrease in the transition temperature and N concentration. The results reveal that the sample grown at 425 °C contains approximately 2 × 1020 cm−3 of N. Thus, efficient nitrogen doping can be achieved through mist CVD. The temperature‐resistance characteristics of VO2 thin films are measured to investigate their electrical properties and MIT temperatures. The results reveal that for undoped samples, the transition temperature slightly decreases with the decrease in the growth temperature. Furthermore, the sample grown at 425 °C exhibits a considerable change in resistance because of MIT at approximately 29.5 °C. These results prove the potential of using mist CVD N‐doped thin films for smart window applications to address future energy problems.

Temperature-dependent epitaxial evolution of carbon-free corundumα-Ga2O3 on sapphire

Abstract Unintentionally doped carbon impurities from organometallic precursors are primary sources of carrier compensation and mobility degradation in wide bandgap semiconductors, leading to lowered performance of power devices. To address this challenge, carbon-free α-Ga2O3 single crystalline thin-films were heteroepitaxially grown on sapphire substrates by using gallium inorganic precursors through mist chemical vapor deposition technique. Determined through temperature dependence of growth rates, three distinct growth regimes are identified: the surface reaction limited regime below 480°C, the mid-temperature mass-transport limited regime (480-530 °C) and the high temperature limited regime related to desorption or phase transition. With an optimized around 530 °C, the densities of screw and edge dislocations are reduced to 7.17×10^6 and 7.60×10^9 cm-2, respectively. Notably, carbon incorporation was eliminated in the α-Ga2O3 grown by inorganic GaCl3, as evidenced by the absence of carbon-related vibrational bands in Raman scattering analysis, while crystalline quality was comparable to that grown with organometallic precursors. The high solubility of GaCl3 in water is expected to enable the rapid growth of high-purity α-Ga2O3 with improved electronic performances. Keywords: ultrawide bandgap semiconductor, chemical vapor deposition, impurity contamination.

Ge doping of α-Ga2O3 thin films via mist chemical vapor deposition and their application in Schottky barrier diodes

We performed Ge doping of α-Ga2O3 thin films grown on m-plane sapphire substrates using mist chemical vapor deposition. Although the typical growth rate was high at 4 μm/h, the resultant α-Ga2O3 thin films exhibited high crystallinity. We controlled the carrier density in the range of 8.2 × 1016–1.6 × 1019 cm−3 using bis[2-carboxyethylgermanium(IV)]sesquioxide as the Ge source. The highest mobility achieved was 66 cm2 V−1 s−1 at a carrier concentration of 6.3 × 1017 cm−3. Through secondary ion mass spectrometry analysis, a linear relationship between the Ge concentration in the α-Ga2O3 thin films and the molar ratio of Ge to Ga in the source solution was established. The quasi-vertical Schottky barrier diode fabricated using the Ge-doped α-Ga2O3 thin films exhibited an on-resistance of 7.6 mΩ cm2 and a rectification ratio of 1010. These results highlight the good performance of the fabricated device and the significant potential of Ge-doped α-Ga2O3 for power-device applications.

Wet and dry etching of ultrawide bandgap LiGa5O8 and LiGaO2

Crystalline thin films of LiGa5O8 have recently been realized through epitaxial growth via mist-chemical vapor deposition. The single crystal, spinel cubic LiGa5O8 films show promising fundamental material properties and, therefore, make LiGa5O8 a potential enabling material for power electronics. In this work, chemical resistance and etch susceptibility were investigated for the first time on crystalline LiGa5O8 thin films with various wet chemistries. It was found that LiGa5O8 is very chemically resistive to acid solutions, with no apparent etching effects observed when placed in concentrated acid solutions of HCl, H2SO4, HF, or H3PO4 at room temperature. In contrast, orthorhombic (010) LiGaO2 shows effective etching in HCl solutions at varying dilution concentrations, with etch rates measured between 8.6 [1000:1 (DI water: HCl concentration)] and 6092 nm/min (37 wt. % HCl). The inductively coupled plasma reactive ion etching (ICP-RIE) of LiGa5O8 using BCl3/Ar and CF4/Ar/O2 gas chemistries was investigated. The etching rate and surface morphology of etched surfaces were examined as a function of RIE and ICP power. Using a CF4/Ar/O2 gas chemistry with an RIE power of 75 W and an ICP power of 300 W resulted in smooth etched planar surfaces while maintaining an etch rate of ∼24.6 nm/min. Similar dry etching studies were performed for LiGaO2. It was found that the BCl3/Ar gas chemistry was better suited for LiGaO2 etching, with similar surface morphology quality being obtained after etching as prior etching when a RIE power of 15 W and an ICP power of 400 W is utilized.

Mist Chemical Vapor Deposition Using Poorly Soluble Particles as Raw Material

AbstractMist chemical vapor deposition (mist‐CVD) is expected to be a potentially low‐environment‐impact and low‐cost crystal growth technique. With mist‐CVD, soluble materials are usually selected as raw materials, and mist is usually generated from an aqueous solution including raw materials. However, any substances to be included in the mist can be potentially supplied as raw materials. Namely, there is no need for the substances to be dissolved in some solvent. Therefore, as a trial, it is demonstrated that mist including poorly soluble particles are used as raw materials for mist‐CVD crystal growth.

Crystallographic and band structure analysis of β-(AlxGa1−x)2O3/β-(InyGa1−y)2O3 thin film grown on β-Ga2O3 substrate via mist CVD

We successfully achieved the growth of a stacked layer composed of β-(AlxGa1−x)2O3/β-(InyGa1−y)2O3 on a (010) β-Ga2O3 substrate using mist chemical vapor deposition (CVD). X-ray diffraction and reciprocal space mapping analyses were conducted, elucidating that the multilayer structure of the β-(AlxGa1−x)2O3/β-(InyGa1−y)2O3 thin film exhibited excellent crystallinity and coherent growth. Scanning transmission electron microscopy further revealed a continuous atomic arrangement at the heterointerface of β-(AlxGa1−x)2O3/β-(InyGa1−y)2O3. Furthermore, the bandgap values of β-(AlxGa1−x)2O3 and β-(InyGa1−y)2O3 thin films were determined to be 5.21 and 4.62 eV, respectively, through electron energy-loss spectroscopy. Notably, a slight broadening was observed in the bandgap transition at the interface of β-(AlxGa1−x)2O3/β-(InyGa1−y)2O3. Energy dispersive x-ray spectroscopy analysis indicated that this phenomenon could be attributed to the diffusion of In into the β-(AlxGa1−x)2O3 thin film layer. These results support mist CVD as a promising growth technique for developing β-Ga2O3-based heterojunction devices.

Fabrication of conductive Nb-doped anatase TiO2 thin films by mist chemical vapor deposition using aqueous solutions of water-soluble Ti and Nb compounds

Electrically conductive Nb-doped anatase TiO2 (Ti1−x Nb x O2: TNO) films can be fabricated through mist CVD using aqueous precursor solutions of water-soluble oxo-peroxo-glycolato titanium complex and ammonium niobium oxalate. Post-deposition annealing in vacuum crystallizes the as-deposited amorphous films into a conductive anatase phase, leading to the fabrication of conductive TNO films. Notably, the addition of H2O2 to the precursor solutions enhances both the crystallinity and conductivity of the annealed TNO films. A Ti0.77N0.23O2 film, annealed at 700 °C, exhibits a resistivity of 2.0 × 10−2 Ω cm at ambient temperature. In general, the solution-based fabrication of TiO2 films relies on organic solvents, which are sometimes toxic and explosive. Here, we demonstrate for the first time that conductive TNO films can be prepared from less toxic and nonflammable aqueous solutions. These findings mark a significant advancement towards a more environmentally compatible process for fabricating TNO films with sufficient conductivity.

Oxygen vacancies modulating performance for Ga2O3 solar-blind photodetectors via low-cost mist chemical vapor deposition

The wide bandgap and excellent chemical and physical properties of gallium oxide (Ga2O3) make it an ideal and natural material for solar-blind photodetectors (SBPDs). However, the development of Ga2O3-based SBPDs in practical applications was hindered by intrinsic oxygen vacancies (VO) in Ga2O3 films, high process requirements, and expensive equipment. Here, VO concentration in the films can be regulated by the introducing of O2 in the carrier gases using the mist chemical vapor deposition (Mist-CVD), which has the benefits of low cost, vacuum-free, and easy procedure. The high-performance Ga2O3 films exhibit a single-crystal tendency and low VO concentration under the Ar (1 L/min) and O2 (0.5 L/min) flow rates at 600 °C. The photodetectors exhibit a large photo-to-dark current ratio of 3.0 × 107, a high specific detectivity of 3.06 ×1011 Jones, and a fast response speed of 0.528/0.158 s at 500 μWcm−2 light intensities at 5 V. The mechanism of VO regulation in Ga2O3 films was demonstrated by adjusting the carrier gas flow rates. We also clarified the effect of VO on the dark current and photocurrents of the devices. The low-cost and straightforward Mist-CVD can provide an effective method to grow high-quality Ga2O3 films, further facilitating their practical application.

Experimental determination of the band offsets at the UWBG p-LiGa5O8/Ga2O3 interface

LiGa5O8, a recently discovered ultrawide bandgap semiconductor exhibiting p-type conductivity at room temperature, is grown on (010) β-Ga2O3 substrate. Utilizing a mist chemical vapor deposition method, LiGa5O8 thin film grown on insulating Ga2O3 substrate exhibits hole concentration ∼2.31 × 1018 cm−3 and hole mobility ∼2.07 cm2/V s. The resulting heterostructure is characterized comprehensively. A smooth, uniform film growth was observed by scanning electron microscopy and atomic force microscopy imaging. Cross-sectional scanning transmission electron microscopy and x-ray diffraction confirm the growth of spinel cubic crystal structure of LiGa5O8 along the [110] direction. Moreover, the valence and conduction band offsets at p-LiGa5O8/β-Ga2O3 heterointerface were determined via x-ray photoelectron spectroscopy. The determined band alignments at LiGa5O8/β-Ga2O3 interface reveal the formation of a type-II (staggered) heterojunction. The experimental measurements provide valuable parameters of the band offsets at the heterointerface, addressing a crucial aspect for future power electronic device design and fabrication based on β-Ga2O3 ultrawide bandgap semiconductors.

Characterization of ZnO Crystals of Various Geometries Grown by Mist Chemical Vapor Deposition on a c‐Plane Sapphire Substrate

ZnO nanowires, nanowalls, and thin‐film‐like crystals are grown simultaneously on a c‐plane sapphire substrate by mist chemical vapor deposition (mist‐CVD) at a high growth temperature. Mist‐CVD is expected to be a potentially low‐environment‐impact and low‐cost crystal growth technique. Using mist‐CVD techniques, a spatial distribution of crystal growth is obtained owing to equipment configurations that enable the simultaneous fabrication of ZnO crystals of various geometries. Their crystallinity and photoluminescence properties are examined to clarify their potential applications and to investigate low‐cost photonic applications based on these properties.

Influence of pH value of precursor on growth, structural, and optical properties of Cu2O thin films grown in Mist-CVD

The Cu2O thin films were prepared from copper (II) acetylacetonate (Cu(acac)2) by the mist chemical vapor deposition (mist-CVD) method, depending on the pH value of the prepared solution. It was confirmed that the well-oriented Cu2O thin films were grown at a dominant (111) peak according to X-ray diffraction (XRD) measurement using an alkaline precursor solution. The pH of the prepared solution drastically affected the surface morphology of the Cu2O thin films, and the lowest Root Means Square (RMS) was found for pH-10 of the precursor solution. The confocal Raman spectrum confirmed the formation of the cubic Cu2O crystal structures for each pH value of the precursor solution. The very transparent Cu2O thin films that were grown. The highest transmittance, 70%, was obtained at 1100 nm from the absorption spectrum for Sample B. The optical energy band gaps of Cu2O thin films were determined using Tauc's method and found to vary within a range of 2.53 to 2.49 eV, indicating a change in the material's electronic structure. The study demonstrated the crucial role of the pH level of the prepared solution in the growth of the Cu2O thin film. Specifically, it was found that a higher pH level resulted in a greater concentration of hydroxide ions (OH−), which was a crucial factor in the growth process. These findings can be significant in using p-type Cu2O thin films prepared based on mist-CVD for optoelectronic applications.

Mist Chemical Vapor Deposition of Bi13S18I2 for Photoelectrochemical-type Photodetection.

Bismuth-based ternary compounds have attracted much attention owing to their various merits, such as low toxicity and tunable electrical and optical properties. However, these compounds are yet to be understood due to the lack of suitable targets limited by immature synthesis techniques. In this work, we aimed at the synthesis, properties investigation, and photodetection application of Bi13S18I2. Mist chemical vapor deposition was adopted for the deposition of the Bi13S18I2 thin film for the first time. The deposition mechanism was discussed from the perspective of crystal phase and surface morphology. Based on the Bi13S18I2 thin film synthesized at optimal temperature, we constructed a photoelectrochemical-type photodetector. The photodetection performance was evaluated from the points of electrolyte composition, working temperature, and bias voltage. This study would pave the way for the controllable synthesis and applications of bismuth-based ternary compounds.

Investigation on the Mist Intensity to Deposit Gallium Oxide Thin Films by Mist Chemical Vapor Deposition

Investigation on the Mist Intensity to Deposit Gallium Oxide Thin Films by Mist Chemical Vapor Deposition

Heteroepitaxial growth of a-, m-, and r-plane α-Ga2O3 thin films on rh-ITO electrodes for vertical device applications

α-Ga2O3 has garnered significant attention as a promising next-generation material for applications in power-switching and deep ultraviolet optoelectronics owing to its ultrawide bandgap. However, fabricating α-Ga2O3-based devices with a vertical structure is challenging because α-Ga2O3 requires heteroepitaxial growth on a heterogeneous substrate, typically insulating sapphire. In this study, we investigated the heteroepitaxial growth of a-, m-, and r-plane α-Ga2O3 on rhombohedral indium tin oxide (rh-ITO) electrodes via mist chemical vapor deposition. X-ray diffraction (XRD) 2θ-ω and φ scan analyses revealed that a-, m-, and r-plane α-Ga2O3 epitaxial thin films were successfully grown on rh-ITO bottom electrodes when α-Fe2O3 buffer layers were inserted. No phase separation or in-plane rotation was observed in the XRD results of the α-Ga2O3 thin films. The α-Fe2O3 buffer layers facilitated the preferential growth of α-Ga2O3 on various planes of rh-ITO. The surface morphology of the a-, m-, and r-plane α-Ga2O3, examined via field emission scanning electron microscopy, differed depending on the crystal plane. The results obtained in this study have potential applications in the development of vertical-structured devices with α-Ga2O3 thin films.

Visible-light absorption of indium oxide thin films via Bi3+ doping for visible-light-responsive photocatalysis

In2O3 is commonly used as a photocatalyst for hydrogen energy storage. However, it has the disadvantage of a large optical bandgap (3.7 eV), resulting in low absorption of visible light, which corresponds to 50 % of sunlight energy. Band engineering is an efficient method to add visible-light responsivity to wide-bandgap In2O3. In this study, the band engineering of In2O3 was performed by incorporating bismuth to obtain an intermediate band that could absorb visible light. Thin films of In2O3 doped with bismuth (IBO) were grown using mist chemical vapor deposition (CVD), and their optical properties were investigated. Transmittance measurements showed that the IBO thin film exhibited absorption in the visible light region of 400–700 nm, indicating that the introduction of bismuth created an intermediate band in the bandgap of In2O3. IBO thin films with platinum as a co-catalyst could almost completely degrade methylene blue under visible light (420 nm) irradiation for 72 h. Thus, the incorporation of bismuth into In2O3 via mist CVD and the strong absorption of visible light owing to its intermediate band were demonstrated. This study demonstrates that IBO thin films with large absorption in the visible-light region are promising materials for visible-light-responsive photocatalysts.

Improved heteroepitaxy of κ-Ga2O3 on c-plane sapphire by initial mist flow stabilization during mist chemical vapor deposition

Kappa-phase gallium oxide (κ-Ga2O3) is a metastable gallium oxide polymorph and an ultrawide bandgap semiconductor with potential applications in various electronic and optoelectronic devices. In this study, we present the effect of mist flow stabilization during the initial stages of mist chemical vapor deposition process on the crystal quality of heteroepitaxial thin films of κ-Ga2O3 grown on c-plane sapphire. Two κ-Ga2O3 films were grown using the same growth parameters with the only differing factor being the stabilization of the initial mist flow. Structural and optical properties of the films were characterized. Both processes resulted in epitaxial growth; however, controlling the initial mist flow leads to an improved Ga2O3/Al2O3 interface and superior crystal quality. The film grown with regulated mist flow exhibited an optical bandgap of 4.93 eV, a smooth surface with a root mean square roughness of 1.56 nm and a rocking curve full-width half-maximum value of 0.17°. Also, Ti/Au deposition on κ-Ga2O3 resulted in an interface resembling that of β-Ga2O3 and Ti/Au, explaining previous reports of ohmic behavior between κ-Ga2O3 and Ti/Au contacts.

Controlled Crystallinity of a Sn-Doped α-Ga2O3 Epilayer Using Rapidly Annealed Double Buffer Layers.

Double buffer layers composed of (AlxGa1-x)2O3/Ga2O3 structures were employed to grow a Sn-doped α-Ga2O3 epitaxial thin film on a sapphire substrate using mist chemical vapor deposition. The insertion of double buffer layers improved the crystal quality of the upper-grown Sn-doped α-Ga2O3 thin films by blocking dislocation generated by the substrates. Rapid thermal annealing was conducted for the double buffer layers at phase transition temperatures of 700-800 °C. The slight mixing of κ and β phases further improved the crystallinity of the grown Sn-Ga2O3 thin film through local lateral overgrowth. The electron mobility of the Sn-Ga2O3 thin films was also significantly improved due to the smoothened interface and the diffusion of Al. Therefore, rapid thermal annealing with the double buffer layer proved advantageous in achieving strong electrical properties for Ga2O3 semiconductor devices within a shorter processing time.